Liquid crystal is a phase between conventional liquid phase and solid crystalline phase, and the matters in such phase has been widely used in thin type display from the middle of the 20th century due to its physical, chemical and optical characteristics. Liquid crystal phase can only be generated by a combination of molecules having special shapes, which may flow like a liquid but also have optical properties of crystal. Recently, the definition of liquid crystal has expanded to cover all matters that can be in liquid crystal phase at a certain range of temperature while being normal crystal in a relatively low temperature. Liquid crystal may be one organic compound, or a composition combined by the intermolecular forces. Liquid crystal have special optical properties and are sensitive to electromagnetic fields, therefore they are of extremely high practical value.
Liquid crystal display materials have obvious advantages as follows: low driving voltage, low power consumption, high reliability, the ability to display more information, full-color, free of flicker, no harm to human body, automatic production, low cost, being useful for variety of sizes and types of liquid crystal display (LCD), easy to carry and the like. Due to these advantages, the sizes of computer terminals and televisions manufactured with these materials may be significantly reduced.
Dielectric anisotropy is the main parameter of a liquid crystal determining the behavior of the liquid crystal molecules in an electric field, if the dielectric constant in the direction parallel to the alignment of a liquid crystal (hereinafter simply referred as “parallel dielectric constant”) is expressed as ∈∥ and the dielectric constant in the direction perpendicular to the alignment of a liquid crystal (hereinafter simply referred as “perpendicular dielectric constant”) is expressed as ∈⊥, the dielectric anisotropy can be expressed as Δ∈=∈∥−∈⊥. When Δ∈>0, the liquid crystal is called positive liquid crystal, otherwise a negative liquid crystal.
In prior art, liquid crystal layer of a LCD generally employs positive liquid crystal. Since the positive liquid crystal has small ∈⊥, resulting in a large incline angle between the liquid crystal molecule and the display panel, which is unfavorable for the rotation of the liquid crystal molecule in the plane parallel to the display panel, therefore, transmittance of the LCD comprising such liquid crystal layer is low.